Conjugated diene rubber gel, rubber compositions containing the same and process for production of conjugated diene rubber

ABSTRACT

A conjugated diene rubber gel comprising 80-99 weight % of conjugated diene monomer units and 20-1 weight % of aromatic vinyl monomer units and having a swelling index of 16 to 70 as measured in toluene. A rubber vulcanizate made from a rubber composition comprising this conjugated diene rubber gel and a rubber capable of being crosslinked with sulfur exhibits good abrasion resistance and low heat-build up without deterioration of mechanical properties, and thus, is suitable for tire materials. This conjugated diene rubber gel can be produced with high efficiency by emulsion-polymerization of a monomer mixture comprising 50-99.9 weight % of a conjugated diene monomer, 0-30 weight % of an aromatic vinyl monomer, 0-20 weight % of a crosslinking monomer.

TECHNICAL FIELD

[0001] This invention relates to a novel conjugated diene rubber gel, arubber composition comprising the conjugated diene rubber gel, and aprocess for producing a conjugated diene rubber. More specifically itrelates to a conjugated diene rubber gel capable of giving a rubbercomposition suitable for a tire having a high abrasion resistance and alow heat-build up, and the rubber composition comprising the conjugateddiene rubber gel, and a process for producing a conjugated diene rubberwith enhanced productivity.

BACKGROUND ART

[0002] In recent years, improvement of various properties is requiredfor a rubber composition used for an automobile tire. Especially,improvement of mechanical properties, abrasion resistance andappropriately reduced rolling resistance (i.e., low heat-build up) isrequired for a rubber composition used for sidewall and bead of anautomobile tire.

[0003] Natural rubber is consumed in a large amount for an automobiletire. In many cases, other rubbers are blended with natural rubber forimproving various properties of natural rubber. In one example, apolybutadiene rubber is blended for improving the abrasion resistance,and, in another example, a styrene-butadiene copolymer rubber is blendedfor Improving the mechanical properties. However, there is a basiccontradiction between properties required for a tire rubber, and thus,all of the properties required for an automobile tire are difficult tosatisfy concurrently. For example, enhancement of abrasion resistance isaccompanied by mechanical properties, and enhancement of mechanicalproperties is accompanied by increase of heat-build up.

[0004] In view of miscibility of a rubber raw material with areinforcing agent or other additives, it is usually desired that arubber raw material does not have a gel structure. However, a rubber gelhaving a gel structure has been proposed for giving a rubber having lowheat-build up and high abrasion resistance. For example, a rubbercomposition comprising a polychloroprene gel was proposed in JapaneseUnexamined Patent Publication (hereinafter abbreviated to “JP-A”) No.H3-37246. This rubber composition gives a rubber exhibiting satisfactorylow heat-build up and high abrasion resistance, but, as thepolychloroprene gel contains chlorine, it causes a problem of damagingthe environment when scrap tires are incinerated, which makes it'spractical use difficult.

[0005] Further, as for a conjugated diene rubber, a rubber compositioncomprising a polybutadiene gel, and a rubber composition comprising astyrene-butadiene copolymer rubber gel were proposed in JP-AH6-57038 andJP-AH10-204217, respectively. These rubber compositions give rubbershaving satisfactory low heat-build up, but the resulting rubbersoccasionally exhibit poor abrasion resistance, reduced breakingelongation and poor mechanical properties.

[0006] An emulsion polymerization procedure is widely employed for theproduction of a conjugated diene rubber. In the production process by anemulsion polymerization procedure, a latex prepared by emulsionpolymerization in a manner such that a desired polymer composition Isobtained is coagulated with an inorganic salt as a coagulant to formcrumbs having a size of about 2 to 10 mm, water is separated therefromand the crumbs are washed and then dried to give an object rubber.However, for example, in the case where a butadiene-styrene copolymerrubber having a high, e.g. about 35 to 50% by weight of, bound styrenecontent is produced by an emulsion polymerization procedure, a latex asobtained by emulsion polymerization exhibits a poor coagulability, andtherefore, there is a tendency such that the latex is partlynon-coagulated and the thus-formed sticky crumbs are bonded together toform unusually large crumbs. Such large crumbs have a problem such thatwater tends to remain inside of each crumb and thus, a substantiallylong period is required for drying and partial drying failure occurs toform a wet spot. Further, the sticky crumbs are liable to be depositedon an inner wall of a coagulating vessel or on stirrer vanes.

[0007] To solve the above-mentioned problems caused by the stickycrumbs, procedures for appropriately adjusting the concentration of aninorganic salt coagulant, the solid content in the latex, thecoagulation temperature and the stirring conditions, or for using apolymeric flocculant or a heat-sensitive coagulant in addition to theinorganic salt coagulant. However, the above-mentioned problems aredifficult or even impossible to completely solve by these procedures.

DISCLOSURE OF THE INVENTION

[0008] In view of the foregoing, a first object of the present inventionis to provide a conjugated diene rubber gel capable of giving a rubbercomposition exhibiting improved abrasion resistance and reducedheat-build up without deterioration of mechanical properties.

[0009] A second object of the present invention is to provide a rubbercomposition exhibiting improved abrasion resistance and reducedheat-build up without deterioration of mechanical properties.

[0010] A third object of the present invention is to provide a processfor producing a conjugated diene rubber gel with enhanced productivity.

[0011] A fourth object of the present invention is to provide a processfor producing a conjugated diene-aromatic vinyl copolymer rubber havinga high bound aromatic vinyl content, which is characterized as formingnon-sticky crumbs and exhibiting enhanced coagulability.

[0012] Thus, in a first aspect of the present invention, there isprovided a conjugated diene rubber gel comprising 80% to 99% by weightof conjugated diene monomer units and 20% to 1% by weight of aromaticvinyl monomer units and having a swelling index in the range of 16 to 70as measured in toluene.

[0013] In a second aspect of the present invention, there is provided arubber composition comprising a conjugated diene rubber gel comprising80% to 99% by weight of conjugated diene monomer units and 20% to 1% byweight of aromatic vinyl monomer units and having a swelling index inthe range of 16 to 70 as measured in toluene, and a rubber capable ofbeing crosslinked with sulfur.

[0014] In a third aspect of the present invention, there is provided aprocess for producing a conjugated diene rubber gel having a swellingindex of not larger than 70 as measured in toluene, characterized inthat a monomer mixture comprising 50% to 99.9* by weight of a conjugateddiene monomer, 0% to 30% by weight of an aromatic vinyl monomer, 0% to20% by weight of other ethylenically unsaturated monomer and 0.1% to 20%by weight of a crosslinking monomer is copolymerized by an emulsionpolymerization procedure.

[0015] In a fourth aspect of the present invention, there is provided aprocess for producing a conjugated diene-aromatic vinyl copolymerrubber, characterized in that a monomer mixture comprising 15% to 69.8%by weight of a conjugated diene monomer, 30.1% to 65* by weight of anaromatic vinyl monomer, 0% to 20% by weight of other ethylenicallyunsaturated monomer and 0.1% to 20% by weight of a crosslinking monomeris copolymerized by an emulsion polymerization procedure.

BEST MODE FOR CARRYING OUT THE INVENTION Conjugated Diene Rnbber Gel

[0016] The conjugated diene rubber gel of the present invention ischaracterized as comprising 80% to 99% by weight, preferably 83% to 95%by weight and more preferably 86% to 90% by weight, of conjugated dienemonomer units and 20% to 1% by weight, preferably 17% to 5% by weightand more preferably 14% to 10% by weight, of aromatic vinyl monomerunits. This conjugated diene rubber gel may be produced either by usingor without use of a crosslinking monomer, but is preferably produced bycopolymerization using a crosslinking monomer. If desired, othercopolymerizable ethylenically unsaturated monomer may be copolymerizedfor the production of the copolymer rubber gel.

[0017] Thus, the conjugated diene rubber gel of the present inventionusually comprises 80 to 99% by weight of conjugated diene monomer units,1% to 20% by weight of aromatic vinyl monomer units, 0% to 19% by weightof other ethylenically unsaturated monomer units and 04 to 1.5% byweight of crosslinking monomer units. A preferable conjugated dienerubber gel of the present invention comprises 83 to 95% by weight ofconjugated diene monomer units, 5% to 17% by weight of aromatic vinylmonomer units, 0% to 5% by weight of other ethylenically unsaturatedmonomer units and 0% to 1% by weight of crosslinking monomer units, Amore preferable conjugated diene rubber gel comprises 86 to 90% byweight of conjugated diene monomer units, 10% to 14% by weight ofaromatic vinyl monomer units, 0% to 1% by weight of other ethylenicallyunsaturated monomer units and 0% to 0.5% by weight of crosslinkingmonomer units.

[0018] If the amount of conjugated diene monomer units in the conjugateddiene rubber gel is too small, a rubber vulcanizate has poor mechanicalproperties. In contrast, if the amount of conjugated diene monomer unitsis too large, a rubber vulcanizate exhibits poor abrasion resistance. Ifthe amount of aromatic vinyl monomer units in the conjugated dienerubber gel is too small, a rubber vulcanizate has poor abrasionresistance. In contrast, if the amount of aromatic vinyl monomer unitsis too large, a rubber vulcanizate exhibits undesirably high heat buildup. If the amount of optional other athylenically unsaturated monomerunits in the conjugated diene rubber gel is too large, a rubbervulcanite having good and balanced mechanical properties, abrasionresistance and low heat-build up is difficult to obtain. The use of acrosslinking monomer is optional, but it is preferably used in an amountsuch that the amount of its units in the resulting conjugated dienerubber gel Is in the range of 0.1% to 1.5% by weight so as to produce arubber vulcanizate having desired mechanical properties, abrasionresistance and low heat-build up and exhibiting a swelling index, shownbelow, as measured in toluene.

[0019] The conjugated diene rubber gel of the present invention ischaracterized as exhibiting a swelling index in the range of 16 to 70 asmeasured in toluene. The toluene swelling index is preferably in therange of 17 to 50, more preferably 19 to 45 and especially preferably 20to 40.

[0020] If the toluene swelling index is too small, a rubber compositionhaving a reinforcing agent incorporated therein tends to exhibit anundesirably large Mooney viscosity and poor processability, and a rubbervulcanizate has poor elongation and poor abrasion resistance. Incontrast, if the toluene swelling index is too large, a rubbervulcanizate has poor abrasion resistance and exhibits undesirably highheat-build up.

[0021] The swelling index of the conjugated diene rubber gel as measuredIn toluene is calculated from the following equation.

Swelling Index=weight of rubber gel as swollen with toluene/weight ofrubber gel as dried

[0022] More specifically the toluene swelling index is determined asfollows. 250 g of a conjugated diene rubber gel is placed in 25 ml oftoluene and the mixture is shaken for 24 hours whereby the rubber gel isswollen. The swollen rubber gel is subjected to centrifuging under acentrifugal force of at least 400,000 m/sec² by a centrifugal separator.The as-centrifuged rubber gel is weighed in a wet state (weight “a”),and then dried at 70° C. to a constant weight. The dried gel is weighed(weight “b”). The toluene swelling index is expressed by the ratio ofweight “a” of wet rubber gel to weight “b” of dried rubber gel.

[0023] The conjugated diene monomer used is not particularly limited,and, as specific examples thereof, there can be mentioned 1,3-butadiene,2-methyl-1,3-butadiene, 1,3-pentadiene and 2-chloro-1,3-butadiene. Ofthese, 1,3-butadiene and 2-methyl-1,3-butadiene are preferable.1,3-butadiene is most preferable. The conjugated diene monomer may beused either alone or in combination.

[0024] The aromatic vinyl monomer used is an aromatic monovinyl compoundand is not particularly limited. As specific examples thereof, there canbe mentioned styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene,2,4-dimethylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene,p-t-butylstyrene, α-methyletyrene, α-methyl-p-methylstyrene,o-ohlorostyrene, m-chlorostyrene, p-chlorostyrene, p-bromostyrene,2-methyl-4,6-dichlorostyrene, 2,4-dibromostyrene and vinylnaphthalene.Of these, styrene is preferable.

[0025] Other ethylenically unsaturated monomers, which are optionallycopolymerized with the conjugated diene monomer and the aromatic vinylmonomer, are not particularly limited, and include α,β-ethylenicallyunsaturated carboxylic acid ester monomers, α,β-ethylenicallyunsaturated nitrile monomers, α, β-ethylenically unsaturated carboxylicacid monomers, α,β-ethylenically unsaturated carboxylic acid amidemonomers and olefin monomers.

[0026] As specific examples of the α,β-ethylenically unsaturatedcarboxylic acid ester monomers, there can be mentioned alkyl esters suchas methyl acrylate, methyl methacrylate, ethyl acrylate, butyl acrylate,2-ethylhexyl acrylate and lauryl methacrylate; alkoxy-substituted alkylesters such as methoxyethyl acrylate and methoxyethoxyethyl acrylate;cyano-substituted alkyl esters such as cyanomethyl acrylate;2-cyanoethyl acryl ate and 2-ethyl-6-cyanohexyl acrylate;hydroxy-substituted alkyl esters such as 2-hydroxyethyl acrylate and2-hydroxyethyl methacrylate; epoxy-substituted alkyl esters such asglycidyl acrylate and glycidyl methacrylate; amino-substituted alkylesters such as N,N′-dimethylaminoethyl acrylate; halogen-substitutedalkyl esters such as 1,1,1-trifluoroethyl acrylate; and complete alkylesters of a polycarboxylic acid such as diethyl maleate, dibutylfumarate and dibutyl itaconate.

[0027] As specific examples of the α,β-ethylenically unsaturated nitrilemonomers, there can be mentioned acrylonitrile and methacrylonitrile.

[0028] As specific examples of the α,β-ethylenically unsaturatedcarboxylic acid monomers, there can be mentioned monocarboxylic acidssuch as acrylic acid and methacrylic acid; polycarboxylic acids such asmaleic acid, fumaric acid and itaconic acid, and partial alkyl esters ofa polycarboxylic acid such as monobutyl fumarate, monobutyl maleate andmonoethyl itaconate.

[0029] As specific examples of the α,β-ethylenically unsaturatedcarboxylic acid amide monomers, there can be mentioned acrylamide,methacrylamide, N,N′-diemthylacrylamide, N-butoxymethylacrylamide,N-butoxymethylmethacrylamide, N-methylolacrylamide andN,N′-dimethylolacrylamide.

[0030] As preferable examples of the olefin monomers, there can bementioned chain-like or cyclic monoolefin compounds having 2 to 10carbon atoms, such as ethylene, propylene, 1-butene, cyclopentene and2-norbornene.

[0031] Further, vinyl chloride, vinylidene chloride and vinylpyridinecan be mentioned as the optional ethylenically unsaturated monomers.

[0032] The above-recited ethylenically unsaturated monomers may be usedeither alone or as a mixture of at least two thereof. The crosslinkingmonomer used for forming a gel structure in enhanced efficiency is acompound having at least two, preferably 2 to 4 carbon-carbon doublebonds capable of being copolymerized with a conjugated diene monomer. Asspecific examples of the crosslinking monomers, there can be mentionedpolyvinyl aromatic compounds such as diisopropenylbenzene,divinylbenzene, trilsopropenylbenzene and trivinylbenzene; unsaturatedester compounds of an α,β-ethylenically unsaturated carboxylic acid,such as vinyl acrylate, vinyl methacrylate and allyl methacrylate;unsaturated ester compounds of a polycarboxylic acid, such as diallylphthalate, triallyl cyanurate, triallyl isocyanurate and triallyltrimellitate; unsaturated ester compounds of a polyhydric alcohol, suchas ethylene glycol diacrylate, ethylene glycol dimethacrylate andpropylene glycol dimethacrylate; and 1,2-polybutadiene, divinyl ether,divinyl sulfone and N,N′-m-phenylene maleimide.

[0033] Further, the crosslinking monomer includes unsaturated polyestercompounds made from a polyhydric alcohol and an unsaturatedpolycarboxylic acid. As specific examples of the polyhydric alcohol,there can be mentioned aliphatic diols and aromatic diols, such asethylene glycol, propylene glycol, butanediol, hexanediol,neopentylglycol and bisphenol A; polyethylene glycol having 2 to 20,preferably 2 to 8 oxyethylene units; and polyols such as glycerine,trimethylolpropane, pentaerythritol and sorbitol. As specific examplesof the unsaturated polycarboxylic acid, there can be mentioned maleicacid, fumaric acid and itaconic acid.

[0034] Of the above-recited crosslinking monomers, divinylbenzene isespecially preferable. The divinylbenzene includes ortho-, meta- andpara-isomeric compounds. These isomeric compounds may be used eitheralone or as a mixture of at least two thereof.

[0035] The conjugated diene rubber gel of the present inventionpreferably has a particle diameter in the range of 5 to 1,000 nm, morepreferably 20 to 400 nm and especially preferably 50 to 200 nm. By theterm “particle diameter” used herein, we mean a weight average particlediameter as determined as follows. The conjugated diene rubber gel isdyed with osmium tetroxide or other appropriate dye. The dyed rubber gelis fixed, and observed by a transmission electron microscope to measurethe particle diameter. The measurement is made on about 100 rubber gelparticles and a weight average particle diameter is calculated.

[0036] The process for producing the conjugated diene rubber gel of thepresent invention is not particularly limited, and includes, forexample, (1) a process wherein the rubber gel is directly produced by anemulsion polymerization procedure using a crosslinking monomer, (2) aprocess wherein an emulsion polymerization is continued until a highconversion, e.g., at least about 90% of conversion, is obtained wherebya gel structure is produced within each latex particle, (3) a rubberlatex having no gel structure, prepared by an emulsion polymerizationprocedure, is treated with a crosslinking compound to effect apost-crosslinking, and (4) a process wherein a rubber gel solution in anorganic solvent, prepared by a solution polymerization procedure, isemulsified in water in the presence of an emulsifier, and then, prior toor after the removal of the organic solvent, the emulsified product istreated with a crosslinking compound to effect a post-crosslinking. Theprocesses (1), (2) and (3) may be carried out either alone or incombination.

[0037] However, to produce the conjugated diene rubber gel of thepresent invention with a high efficiency, process (1) is preferablyadopted wherein the rubber gel is directly produced by an emulsionpolymerization procedure using a crosslinking monomer. In the case wherethe rubber gel is directly produced by an emulsion polymerizationprocedure, the amount of crosslinking monomer, the amount of chaintransfer agent and the conversion upon termination of polymerization canbe appropriately chosen so that the desired toluene swelling index isobtained.

[0038] In the case where the conjugated diene rubber gel of the presentinvention is directly produced by an emulsion polymerization procedureusing a crosslinking monomer, the amount of crosslinking monomer isusually in the range of 0.1% to 1.5% by weight, preferably 0.1% to 1% byweight and more preferably 0.2* to 0.5% by weight, based on 100% byweight of the total monomers. In this case using a crosslinking monomer,the resulting conjugated diene rubber gel comprises 80% to 98.9% byweight, preferably 83% to 94.9% by weight and more preferably 86% to89.8% by weight, of conjugated diene monomer units, 1% to 19.9% byweight, preferably 5% to 16.9% by weight and more preferably 10% to13.8% by weight, of aromatic vinyl monomer units, 0% to 19% by weight,preferably 0% to 5% by weight and more preferably 0% to 1% by weight, ofother ethylenically unsaturated monomer units and 0.1% to 1.5% byweight, preferably 0.1% to 1% by weight and more preferably 0.2% to 0.5%by weight, of crosslinking monomer units.

[0039] As specific examples of the crosslinking compound used forpost-crosslinting conjugated diene rubber latex particles, there can bementioned organic peroxides such as dicumyl peroxide,t-butylcumylperoxide, bis-(t-butyl-peroxy-isopropyl)benzene, di-t-butylperoxide, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide and t-butylperbenzoate; organic azo compounds such as azobisisobutyronitrile andazobiscyclohxanenitrile: and dimereapto compounds and polymercaptocompounds, such as dimercaptoethane, 1,6-dimercaptohexane and 1,3,5-trimercapto-triazine. Of these, organic peroxides are preferable.

[0040] The reaction conditions for post-crosslinking vary depending uponthe reactivity and amount of crosslinking compound, but, a reactionpressure in the range of normal pressure to high pressure (i.e., about 1MPa), a reaction temperature in the range of room temperature to about170° C. and a reaction time in the range of 1 minute to 24 hours canappropriately be chosen. The kind and amount of crosslinking monomer,and reaction conditions are chosen so that the desired toluene swellingindex is obtained.

Process for Producing Conjugated Diene Rubber

[0041] The process for producing a conjugated diene rubber according tothe present invention includes the following two processes (1) and (2).In a process (1), an emulsion copolymerization is carried out withoutuse of or by using a small amount of an aromatic vinyl monomer toproduce a conjugated diene rubber gel, namely, a monomer mixturecomprising 50% to 99.9% byweight of a conjugateddienemonomer, 0% to 30%byweight of an aromatic vinyl monomer, 0% to 20% by weight of otherethylenically unsaturated monomer and 0.1% to 20% by weight of acrosslinking monomer is copolymerized by an emulsion polymerizationprocedure to obtain a conjugated diene rubber gel having a swellingindex of not larger than 70 as measured intoluene (this process (1) Ishereinafter referred to as “first production process”). In a process(2), an emulsion copolymerization is carried out by using a relativelylarge amount of an aromatic vinyl monomer to produce a conjugated dienerubber, namely, a monomer mixture comprising 15% to 69.86% by weight ofa conjugated diene monomer, 30.1% to 65% by weight of an aromatic vinylmonomer, 0% to 20% by weight of other ethylenically unsaturated monomerand 0.1% to 20% by weight of a crosslinking monomer is copolymerized byan emulsion polymerization procedure to produce a conjugateddiene-aromatic vinyl copolymer rubber (this process (2) is hereinafterreferred to as “second production process”).

[0042] First, the first production process will be described in detail.

[0043] The monomer composition used in the first production processcomprises 50% to 99.9% by weight, preferably 70% to 94.9% by weight,more preferably 74% to 89.9% by weight and especially preferably 79.5%to 85.8% by weight of a conjugated diene monomer, 0% to 30t by weight,preferably 5% to 28% by weight, more preferably 10% to 25% by weight andespecially preferably 14% to 20% by weight of an aromatic vinyl monomer,0% to 20% by weight, preferably 0% to 5% by weight and more prefarably0% to 1% by weight of other ethylenically unsaturated monomer, and 0.1%to 20% by weight, preferably 0.1% to 2% by weight, more preferably 0.1to 1% by weight and especially preferably 0.2% to 0.5% by weight ofacrosslinking monomer.

[0044] If the amount of conjugated diene monomer is too small, a rubbervulcanizate has poor mechanical properties. In contrast, if the amountof conjugated diene monomer is too large, a rubber vulcanizate has poorabrasion resistance. If the amount of aromatic vinyl monomer is toosmall, a rubber vulcanizate has poor abrasion resistance. In contrast,if the amount of aromatic vinyl monomer is too large, a rubbervulcanizate exhibits undesirably high heat-build up. If the amount ofoptional other ethylenically unsaturated monomer is too large, a rubbervulcanizate having good and balanced mechanical properties, abrasionresistance and low heat-build up is difficult to obtain. If the amountof crosslinking monomer is too small, a rubber vulcanizate has poorabrasion resistance and exhibits high heat-build up. In contrast, if theamount of crosslinking monomer is large, a rubber composition having areinforcing agent incorporated therein has a large Mooney viscosity andpoor processability, and a rubber vulcanizate has poor abrasionresistance.

[0045] As specific examples of the conjugated diene monomer, thearomatic vinyl monomer, the other ethylenically unsaturated monomer andthe crosslinking monomer, there can be mentioned those which arehereinbefore recited as monomers constituting the conjugated dienerubber gel.

[0046] The procedures and conditions adopted for emulsion polymerizationare not particularly limited. Conventional emulsifiers, polymerizationinitiators, chain transfer agents, polymerization terminators andantioxidants can be used for emulsion polymerization.

[0047] The emulsifier used is not particularly limited and includesfatty acid soaps and rosin soaps. As specific examples of the fatty acidsoaps, there can be mentioned sodium salts and potassium salts of a longchain fatty acid having 12 to 18 carbon atoms such as lauric acid,myristic acid, palmitic acid, stearic acid, oleic acid or a mixed fattyacid thereof. As specific examples of the rosin soaps, there can bementioned sodium salts and potassium salts of a disproportionated orhydrogenated product of natural rosin such as gum rosin, wood rosin ortall oil rosin. Any particular limitation is imposed on the amount ofemulsifier, but, its amount is usually in the range of 0.05 to 15 partsby weight, preferably 0.5 to 10 parts by weight and more preferably 1 to5 parts by weight, based on 100 parts by weight of the monomers.

[0048] The polymerization initiator includes, for example, hydrogenperoxide, an organic peroxide, a persulfate and an organic azo compound,and redox catalysts comprising a combination thereof with ferric sulfateor sodium formaldehyde sulfoxylate.

[0049] As specific examples of the organic peroxide, there can bementioned dicumyl peroxide, t-butylcumyl peroxide,bis-(t-butyl-peroxy-isopropyl)benzene, di-t-butyl peroxide, benzoylperoxide, 2,4-dichlorobenzoyl peroxide and t-butyl perbenzoate. Asspecific examples of the persulfate, there can be mentioned ammoniumpersulfate, sodium persulfate and potassium persulfate. As specificexamples of the organic azo compound, there can be mentionedazobisisobutyronitrile and azobiscyclohexanenitrile. The amount ofpolymerization initiator is usually in the range of about 0.001 to 1part by weight based on 100 parts by weight of the monomers, and can beappropriately chosen so that a desired reaction rate is obtained at adesired reaction temperature.

[0050] As specific examples of the chain transfer agent, there can bementioned mercaptans such as 2,4,4-trimethylpentane-2-thiol,2,2,4,6,6-pentamethylheptane-4-thiol, 2,2,4,6,6,8,8-heptamethylnonane-4-thiol, t-dodecyl mercaptan and t-tetradecylmercaptan: xanthogen disulfides such as dimethylxanthogen disulfide,diethylxanthogen disulfide and diisopropylxanthogen disulfide; thiuramdisulfides such as tetramethylthiuram disulfide, tetraethylthiuramdisulfide and tetrabutylthiuram disulfide; halogenated hydrocarbons suchas carbon tetrachloride and ethylene bromide; hydrocarbons such aspentaphenylethane; and terpinolene, α-terpinene, γ-terpinene, dipentene,α-methylstyrene dimer (preferably containing at least 50% by weight of2,4-diphenyl-4-methyl-1-pentene), and 2,5-dihydrofuran. These chaintransfer agents may be used either alone or as a combination of at leasttwo thereof. The amount of chain transfer agent is usually not largerthan 3 parts by weight, preferably in the range of 0.05 to 1 part byweight and more preferably 0.1 to 0.6 part by weight based on 100 partsby weight of the monomers.

[0051] The polymerization terminator is not particularly limited andconventional polymerization terminators can be used. As specificexamples thereof, there can be mentioned polymerization terminatorshaving an amine structure such as hydroxylamine, sodiumdimethyldithiocarbamate, diethylhydroxylamine, and hydroxylaminesulfonicacid and its alkali metal salt; polymerization terminators having noamine structure such as aromatic hydroxydithiocarboxylic acids such ashydroxydimethylbenezenedithiocarboxylic acid,hydroxydiethylbenezenedithiocarboxylic acid andhydroxydibutylbenezenedithiocarboxylic acid, and alkali metal saltsthereof; and hydroquinone derivatives and catechol derivatives. Thesepolymerization terminators may be used either alone or as a combinationof at least two thereof. The amount of polymerization terminator is notparticularly limited but is usually in the range of 0.1 to 10 parts byweight based on 100 parts by weight of the monomers.

[0052] As specific examples of the antioxidant, there can be mentionedhindered phenol compounds such as 2,6-di-t-butyl-4-methylphenol and2,6-di-t-butyl-4-ethylphenol; and hindered amine compounds such asdiphenyl-p-phenylenediamine andN-isopropyl-N′-phenyl-p-phenylenediamine. The amount of antioxidant isusually in the range of 0.05 to 5 parts by weight based on 100 parts byweight of a polymer produced by emulsion polymerization.

[0053] The ratio by weight of the monomers to water in an emulsionpolymerization system is usually in the range of 5/95 to 50/50,preferably 10/90 to 40/60 and more preferably 20/80 to 35/65. If therelative ratio of the monomers Is high, undesirable coagulation tends tooccur and heat of polymerization is difficult to remove. In contrast, ifthe relative ratio of the monomers is low, the productivity is low.

[0054] The polymerization temperature is usually in the range of −5° C.to 80° C., preferably 0° C. to 60° C., more preferably 3° C. to 30° C.and especially preferably 5° C. to 15° C. Too low polymerizationtemperature leads to reduction in cost and productivity. In contrast,too high polymerization temperature gives a rubber vulcanizate havingpoor abrasion resistance and exhibiting high heat-build up.

[0055] The conversion upon the termination of polymerization ispreferably in the range of 50* to 90%, more preferably 60% to 85% andespecially preferably 65% to 80%. If this conversion is too low, theproductivity is low. In contrast, too high conversion gives a rubbervulcanizate having poor abrasion resistance and exhibiting highheat-build up, It is especially preferable to carry out thepolymerization at a temperature of 3° C. to 30° C., and terminate thepolymerization at a conversion of 60% to 85%.

[0056] In the case where the conjugated diene rubber gel is produced byemulsion polymerization, a conventional emulsion polymerizationprocedure can be adopted and, when the conversion reaches apredetermined value, a polymerization terminator Is added to terminatethe polymerization reaction. If desired, an antioxidant is added, andthen residual monomers are removed by heating or steam distillation.Then, a conventional coagulant used in emulsion polymerization is addedto coagulate a copolymer latex and a copolymer is recovered. As thecoagulant, for example, inorganic salts such as calcium chloride, sodiumchloride and aluminum sulfate, and a polymeric flocculant and aheat-sensitive coagulant can be used. The recovered copolymer is washedwith water and then dried to give an object conjugated diene rubber gel.If desired, when the latex is coagulated, an extender oil can be addedto give an oil-extended rubber gel.

[0057] According to the need, prior to coagulation of a latex of theconjugated diene rubber gel, a rubber latex substantially having no gelstructure or a rubber gel other than that of the present invention canbe incorporated in the latex of conjugated diene rubber gel. Thethus-obtained latex mixture is coagulated, and a polymer composition isrecovered. The recovered polymer composition Is dried to give a rubbercomposition comprising a predetermined amount of the conjugated dienerubber gel.

[0058] The composition of the conjugated diene rubber gel produced bythe process of the present invention varies depending upon thecomposition of a monomer mixture charged and the conversion upontermination of polymerization. This is because the monomers usually havedifferent reactivity in emulsion copolymerization. However, a desiredcomposition of the conjugated diene rubber gel can be obtained bypreviously determine the composition of a monomer mixture charged andthe conversion upon termination of polymerization.

[0059] The composition of the conjugated diene rubber gel can bedetermined by NMR analysis, infrared spectrophotometry, ultravioletspectrophotometry, elemental analysis and refractive index measurement.These analyzing procedures may be adopted alone or in combination. Note,in the case when a styrene-butadiene copolymer rubber gel contains asmall amount of bound divinylbenzene units, the content of bounddivinylbenzene units is very difficult to determine by these analyzingprocedures. However, the content of bound divinylbenzene units can bedetermined by calculating the difference between the amount of chargedmonomer and the amount of unreacted monomer in a polymerization mixtureas measured after termination of polymerization.

[0060] The particle diameter of the conjugated diene rubber gel can becontrolled by the ratio of monomer/water in an emulsion copolymerizationsystem, the kind and amount of emulsifier, the kind and amount ofpolymerization initiator, and the polymerization temperature.

[0061] The toluene swelling index of the conjugated diene rubber gel canbe controlled by the amount of crosslinking monomer, the amount of chaintransfer agent and the conversion upon termination of polymerization. Bythe first production process, a conjugated diene rubber gel having aswelling index of not larger than 70 as measured in toluene can easilybe produced with enhanced productivity.

[0062] Secondly, the second production process will be described indetail.

[0063] The monomer composition used in the second production processcomprises 15% to 69.8% by weight, preferably 33% to 64.9% by weight,more preferably 39% to 54.8% by weight, and especially preferably 41.4%to 51.7% by weight of a conjugated diene monomer, 30.1% to 65% byweight, preferably 35% to 62% by weight, more preferably 45% to 60% byweight and especially preferably 48% to 58% by weight of an aromaticvinyl monomer, 0% to 20% by weight, preferably 0% to 5% by weight andmore preferably 0% to 1% by weight of other ethylenically unsaturatedmonomer and 0.1% to 20% by weight, preferably 0.1% to 5% by weight, morepreferably 0.2% to 1% by weight and especially preferably 0.3% to 0.6%by weight of a crosslinking monomer.

[0064] If the amount of conjugated diene monomer is too small or theamount of aromatic vinyl monomer is too large, the resulting copolymerrubber has undesirably high glass transition temperature and is notpreferable. In contrast, if the amount of conjugated diene monomer istoo large or the amount of aromatic vinyl monomer is too small, theresulting copolymer rubber gives a tire having poor road-grippingcharacteristics, and gives a rubber .rulcan zate having undesirably highhardness. If the amount of other ethylenically unsaturated monomer istoo large, a desired rubber having good and balanced properties isdifficult to produce. If the amount of crosslinking monomer is toosmall, a latex has poor coagulability and crumbs are sticky. Incontrast, if the amount of crosslinking monomer is too large, anunreacted crosslinking monomer is in many cases difficult to removeafter termination of polymerization because most crosslinking monomershave a high boiling point.

[0065] The conjugated diene monomer, the aromatic vinyl monomer, theother ethylenically unsaturated monomer and the crosslinking monomer arenot particularly limited, and, as specific examples thereof, there canbe mentioned those which are the same as used in the first productionprocess and hereinbefore recited as monomers constituting the conjugateddiene rubber gel.

[0066] The emulsifier, polymerization initiator, chain transfer agent,polymerization initiator and antioxidant, and the procedures andconditions adopted for emulsion copolymerization may be the same asthose which are explained in the first production process.

[0067] It is especially preferable in the second production process tocarry out the polymerization at a temperature of −5° C. to 80° C., andterminate the polymerization at a conversion of 50% to 90%.

[0068] In view of the object of producing a copolymer rubbercharacterized in that a latex has good coagulability and crumbs are notsticky in the second production process, it is to be noted that, whenthe polymerization temperature is high and the conversion upontermination of polymerization is high, coarse coagulated products havinga size larger than the particle diameter of the desired latex are liableto be produced, and the resulting rubber vulcanizate tends to have poormechanical strengths.

[0069] The procedures for coagulation in the second production processwill now be described.

[0070] The coagulating agent used includes conventional inorganic metalsalts, polymeric flocculants and heat-sensitive coagulating agents. Asspecific examples of the inorganic metal salts, there can be mentionedunivalent metal salts such as sodium chloride, potassium chloride,sodium nitrate, sodium sulfate and sodium carbonate; divalent metalsalts such as calcium chloride, magnesium chloride, calcium sulfate andmagnesium sulfate; and trivalent metal salts such as aluminum chloride,aluminum nitrate and aluminum sulfate. Of these, calcium chloride ispreferable. The amount of inorganic metal salt is usually in the rangeof 0.1 to 100 parts by weight, preferably 1 to 50 parts by weight andmore preferably 2 to 10 parts by weight, based on 100 parts by weight ofthe copolymer rubber in the latex.

[0071] As specific examples of the polymeric flocculants, there can bementioned nonionic, anionic or cationic acrylamide polymers, alkalimetal salts of an anionic acrylic acid polymer, and cationic condensateresins. Of these, cationic condensate resins are preferable. The amountof polymeric flocculant is usually in the range of 0.05 to 10 parts byweight, preferably 0.2 to 5 parts by weight and more preferably 0.4 to 2parts by weight, based on 100 parts by weight of the copolymer rubber inthe latex.

[0072] As specific examples of the heat-sensitive coagulants, there canbe mentioned a polyoxyethylene adduct of an alkylphenol-formaldehydecondensate, a polyoxypropylene adduct thereof and apoly(oxyethylene-oxypropylene) adduct thereof; and a polyoxyethylenealkyl ether, a polyoxyethylene fatty acid ester, a polyoxyethylenesorbitan fatty acid ester, and an oxyethylene-oxypropylene blockcopolymer. Of these, a poly(oxyethylene-oxypropylene) adduct of analkylphenol-formaldehyde condensate is preferable. The amount ofheat-sensitive coagulant is usually in the range of 0.01 to 5 parts byweight, preferably 0.05 to 2 parts by weight, based on 100 parts byweight of the copolymer rubber in the latex.

[0073] The above recited coagulants may be used either alone or incombination. When a heat-sensitive coagulant is used, an inorganic metalsalt is preferably used in combination therewith.

[0074] When a latex is coagulated, an inorganic acid such ashydrochloric acid, nitric acid or sulfuric acid, or an organic acid suchas acetic acid or alkylsulfuric acid is preferably used in addition tothe coagulant. This is because coagulability is more enhanced by theaddition of acid to render the latex acidic, especially adjust the pHvalue in the range of 2 to 5. The above-recited coagulants and acids areadded preferably as an aqueous solution in an amount of 1 to 20 times,more preferably 2 to 15 times and especially preferably 3 to 10 times byweight of the amount of latex. When the amount of water is too small,uncoagulated latex tends to remain within crumbs. In contrast, when theamount of water is too large, coagulability is liable to be reduced.

[0075] When the latex is coagulated, it preferably has a solid contentin the range of 1% to 30% by weight, more preferably 3% to 20% by weightand especially prefarably 5% to 15% by weight. When the solid content istoo small, the productivity is low. In contrast, when the solid contentis too large, uncoagulated latex tends to remain within crumbs. When thelatex is coagulated, its temperature is usually maintained in the rangeof 10° C. to 100° C., preferably 40° C. to 90° C. and more preferably50° C. to 80° C. As the coagulation procedure, a procedure wherein alatex having a predetermined solid concentration is added to an aqueoussolution containing a coagulant and an acid in a predeterminedconcentration is adopted. The addition of latex may be either batchwiseor continuous.

[0076] By the second production process, a conjugated diene-aromaticvinyl copolymer rubber comprising preferably 35% to 55% by weight, morepreferably 40% to 50% by weight of aromatic vinyl monomer units, andhaving a Mooney viscosity in the range of 30 to 300, more preferably 40to 250 and especialy preferably 50 to 200 is easily produced.

[0077] The conjugated diene-aromatic vinyl copolymer rubber is useful asrubber material for tires, cable coverings, hoses, transmission belts,conveyor belts, roll covers, shoe soles, sealing rings and vibrationrubber insulators. Further, this copolymer rubber is suitable for animpact modifier for resins, an additive for adhesives, and a binder forabrasives used for working tools.

Rubber Composition

[0078] The rubber composition of the present invention comprises theabove-mentioned conjugated diene rubber gel, namely, a conjugated dienerubber gel comprising 80t to 99% by weight of conjugated diene monomerunits and 20% to 1% by weight of aromatic vinyl monomer units and havinga swelling index in the range of 16 to 70 as measured in toluene, and arubber capable of being crosslinked with sulfur. A preferable monomercomposition for the conjugated diene rubber gel and a preferable tolueneswelling index of the rubber gel are as described above.

[0079] The rubber capable of being crosslinked with sulfur is notparticularly limited, but is usually chosen from those which have doublebonds in an amount corresponding to an iodine value of at least 2,preferably in the range of 5 to 470. The iodine value is generallydetermined by measuring the degree of unsaturation by adding iodinechloride according to an glacial acetic acid immersion test, and isexpressed by the amount in grams of iodine chemically bound to 100 g ofrubber. The rubber capable of being crosslinked with sulfur usually hasa Mooney viscosity (ML₁₊₄, 100° C.) in the range of 10 to 150,preferably 20 to 120.

[0080] As specific examples of the rubber capable of being crosslinkedwith sulfur, there can be mentioned natural rubber, syntheticpolyisoprene, polybutadiene, an alkyl acrylate-butadiene copolymer, astyrene-butadiene copolymer, a styrene-isoprene copolymer, astyrene-isoprene-butadiene copolymer, an acrylonitrile-butadienecopolymer, a partially hydrogented product of acrylonitrile-butadienecopolymer, an isobutylene-isoprene copolymer an ethylene-propylene-dienecopolymer, and mixtures of these rubbers. These rubbers may bepreviously extended with an extender oil.

[0081] Of these, natural rubber, synthetic polyisoprene, anemulsion-polymerized or solution-polymerized styrene-butadiene copolymercomprising 1% to 60% by weight, preferably 20% to 55% by weight and morepreferably 20% to 50% by weight, of styrene units, polybutadiene havinga high, e.g., at least 90% by weight of, cis-1,4-bond content, andmixtures of these rubbers are pref erable. Natural rubber, syntheticpolyisoprene, the styrene-butadiene copolymer and mixtures of theserubbers are especially preferable.

[0082] The ratio by weight of the conjugated diene rubber gel to therubber capable of being crosslinked with sulfur in the rubbercomposition of the present invention is preferably in the range of 1/99to 50/50, more preferably 5/95 to 40/60 and especially preferably 10/90to 30/70. If the relative amount of conjugated diene rubber gel is toosmall, a rubber vulcanizate has poor abrasion resistance. In contrast,if the relative amount of conjugated diene rubber gel is too large, arubber vulcanizate has poor elongation and exhibits unsatisfactoryheat-build up.

[0083] A reinforcing agent and other ingredients may be incorporated inthe rubber composition of the present invention. The reinforcing agentpreferably includes carbon black and silica.

[0084] As specific examples of carbon black, there can be mentionedfurnace black, acetylene black, thermal black, channel black andgraphite. These carbon blacks may be used either alone or as acombination of at least two thereof.

[0085] The specific surface area of carbon black is not particularlylimited, but, the nitrogen adsorption specific surface area (N₂SA) ispreferably such that the lower limit is 5 m²/g more preferably 50 m²/g,ad the upper limit is 200 m²/g, more preferably 100 m²/g. When thenitrogen adsorption specific surface area is in this range, a rubbervulcanizate has good mechanical properties and abrasion resistance. Thedibutyl phthalate (DBP) adsorption of carbon black is preferably suchthat the lower limit is 5 ml/100 g, more preferably 50 ml/100 g, and theupper limit is 400 ml/100 g, more preferably 200 ml/100 g. When the DBPadsorption is in this range, a rubber vulcanizate has good mechanicalproperties and abrasion resistance.

[0086] The kind of silica is not particularly limited and includes dryprocess white carbon, wet process white carbon, colloidal silica andprecipitated silica described in JP-A S62-62838. Of these, wet processwhite carbon predominantly comprised of silicic acid hydrate ispreferable. These silica may be used either alone or as a combination ofat least two thereof.

[0087] The specific surface area of silica is usually such that thenitrogen adsorption specific surface area as determined by BET method isnot larger than 400 m²/g. The nitrogen adsorption specific surface areais determined by the BET method according to ASTM D-3037-81. The silicapreferably has a pH value smaller than 7.0, preferably in the range of5.0 to 6.9.

[0088] In the case when the rubber composition of the present inventioncomprises silica as a reinforcing agent, a silane coupling agent can bepreferably added to more reduce the heat build-up and more enhance theabrasion resistance.

[0089] The silane coupling agent is not particularly limited, and, asspecific examples thereof, there can be mentioned vinyltriethoxysilane,β-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane,N-(β-amoinoethyl)-γ-aminopropyltrimethoxysilane,bis(3-(triethoxysilyl)propyl)-tetrasulfide,bis(3-(triethoxysilyl)propyl)disulfide, and tetrasulfides described inJP-A H6-248116 such asγ-trimethoxysilylpropyldimethylthiocarbamyltetrasulfide andγ-trimethoxysilylpropylbenzothiazyltetrasulfide. Preferable silanecoupling agents have not larger than 4 sulfur atoms in the molecule toavoid scorch at kneading.

[0090] These silane coupling agent may be used either alone or as acombination of at least two thereof. The amount of silane coupling agentis preferably such that the lower limit is 0.1 part by weight, morepreferably 1 part by weight and especially preferably 2 parts by weight,and the upper limit is 30 parts by weight, more preferably 20 parts byweight and especially preferably 10 parts by weight, based on 100 partsby weight of silica.

[0091] The amount of the reinforcing agent is preferably such that thelower limit is 10 parts by weight, more preferably 20 parts by weightand especially preferably 30 parts by weight, and the upper limit is 200parts by weight, more preferably 150 parts by weight and especiallypreferably 100 parts by weight, based on 100 parts by weight of thetotal rubber ingredients, i.e., the sum of the conjugated diene rubbergel and the rubber capable of being crosslinked with sulfur.

[0092] In the case when silica and carbon black are added in combinationas a reinforcing agent in the rubber composition of the presentinvention, the mixing ratio thereof is appropriately chosen dependingupon the use and object, but the ratio by weight of silica/carbon blackis preferably In the range of 10/90 to 99/1, more preferably 20/80 to95/5 and especially preferably 30/70 to 90/10.

[0093] In addition to the above-mentioned ingredients, ingredients otherthan the reinforcing agents can also be conventionally incorporated inthe rubber composition of the present invention. That is, an appropriateamount of ingredients such as a crosslinking agent, a crosslinkingaccelerator, an accelerator activator, an antioxidant, an activator, aprocess oil, a plasticizer, a lubricant and a filler can beincorporated.

[0094] The crosslinking agent Is not particularly limited, and, asspecific examples thereof, there can be mentioned sulfur such as powderysulfur, precipitated sulfur, colloidal sulfur, insoluble sulfur andhighly dispersible sulfur; halogenatad sulfur such as sulfurmonochloride and sulfur dichloride; organic peroxides such as dicumylperoxide and di-t-butyl peroxide; quinone dioximes such as p-quinonedioxime and p,p′-dibenzoylquinone dioxime; organic polyamine compoundssuch as triethylenetetramlne, hexamethylenediamine carbamate and4,4′-methylenebis-o-chloraniline; and alkylphenol resins having amethylol group. Of these, sulfur is preferable. Powdery sulfur isespecially preferable. These crosslinking agents may be used eitheralone or as a combination of at least two thereof.

[0095] The amount of crosslinking agent is preferably such that thelower limit is 0.1 part by weight, more preferably 0. 3 part by weightand especially preferably 0.5 part by weight, and the upper limit is 15parts by weight, more preferably 10 parts by weight and especiallypreferably 5 parts by weight, based on 100 parts by weight of the totalrubber ingredients. When the amount of crosslinking agent is in thisrange, a rubber vulcanizate has low heat-build up, excellent mechanicalproperties and enhanced abrasion resistance.

[0096] As specific examples of the crosslinking accelerator, there canbe mentioned sulfenamide crosslinking accelerators such asN-cyclohexyl-2-benozothizolsulfenamide,N-t-butyl-2-benozothizolsulfenamide,N-oxyethylene-2-benozothizolsulfenamide andN,N′-diisopropyl-2-benozothizolsulfenamide; guanidine crosslinkingaccelerators such as diphenylguanidine, diorthotolylguanidine andorthotolylbiguanidine; thiourea crosslinking accelerators such asdiethylthiourea; thiazole crosslinking accelerators such as2-mercaptobenzothiazole, dibenzothiazyl disulfide and2-mercaptobenzothiazole zinc salt; thiuram crosslinking acceleratorssuch as tetramethylthiuram monosulfide and tetramethylthiuram disulfide,dithiocarbamate crosslinking accelerators such as sodiumdimethyldithiocarbamate and zinc diethyldithiocarbamate; andxanthogenate crosslinking accelerators such as sodiumisopropylxanthogenate, zinc isopropylxanthogenate and zincbutylxanthogenate.

[0097] The crosslinking accelerator may be used either alone or as acombination of at least two thereof. Of the above-recited crosslinkingaccelerators, a sulfenamide crosslinking accelerator Is preferable. Theamount of crosslinking accelerator is preferably such that the lowerlimit is 0.1 part by weight, more preferably 0.3 part by weight andespecially preferably 0.5 part by weight, and the upper limit is 15parts by weight, more preferably 10 parts by weight and especiallypreferably 5 parts by weight, based on 100 parts by weight of the totalrubbers.

[0098] The accelerator activator is not particularly limited, and, asspecific examples thereof, there can be mentioned higher fatty acidssuch as stearic acid, and zinc oxide. The zinc oxide preferably has highsurface activity with a particle diameter of not larger than 5 μm, and,as specific examples thereof, there can be mentioned active zinc whitehaving a particle diameter in the range of 0.05 to 0.2 μm, and zincwhite having a particle diameter in the range of 0.3 to 1 μm. The zincoxide may be surface-treated with an amine dispersant or wetting agent.

[0099] The accelerator activator may be used either alone or as acombination of at least two thereof. The amount of crosslinkingaccelerator is appropriately chosen depending upon the particular kindthereof. The amount of higher fatty acid is preferably such that thelower limit is 0.05 part by weight, more preferably 0.1 part by weightand especially preferably 0.5 part by weight, and the upper limit is 15parts by weight, more preferably 10 parts by weight and especiallypreferably 5 parts by weight, based on 100 parts by weight of the totalrubbers. The amount of zinc oxide is preferably such that the lowerlimit is 0.05 part by weight, more preferably 0.1 part by weight andespecially preferably 0.5 part by weight, and the upper limit is 10parts by weight, more preferably 5 parts by weight and especiallypreferably 2 parts by weight, based on 100 parts by weight of the totalrubbers. When the amount of accelerator activator is In this range, anuncrosslinked rubber composition has good processability and a rubbervulcanizate has excellent mechanical properties and abrasion resistance.

[0100] Activators such as silicone oil having a functional group such asan epoxy group or an alkoxysilyl group, diethylene glycol andpolyethylene glycol; fillers such as calcium carbonate, talc and clay,and wax can be incorporated in the rubber composition.

[0101] Further, provided that the effect of the present invention can beachieved, the rubber composition of the present invention may comprise ahomopolymer or copolymer having no conjugated diene units which is madefrom at least one monomer selected from epichlorohydrin, ethylene oxide,propylene oxide and allyl glycidyl ether, an acryl rubber, afluororubber, a silicone rubber, an ethylene-propylene rubber and anurethane rubber.

[0102] The rubber composition of the present invention can be preparedby kneading together the respective ingredients according to theconventional procedure, For example, the rubber ingredients andingredients other than a crosslinking agent and a crosslinkingaccelerator are first kneaded together, and then, a crosslinking agentand a crosslinking accelerator are added to the kneaded mixture toobtain the rubber composition. The first kneading of the rubberingredients and ingredients other than a crosslinking agent and acrosslinking accelerator is preferably carried out at a temperature suchthat the lower limit is 80° C., more prefeably 100° C. and especiallypreferably 120° C., and the upper limit is 200° C., more prefeably 190°C. and especially preferably 180° C., and for a time such that the lowerlimit is 30 seconds, more preferably 1 minute, and the upper limit is 30minutes. The second kneading of a crosslinking agent and a crosslinkingaccelerator with the first-kneaded mixture is usually carried out afterthe kneaded mixture is cooled to a temperature of not higher than 100°C., preferably not higher than 80° C.

[0103] The rubber composition of the present invention is usually usedas a rubber vulcanizate.

[0104] The procedur for crosslinking the rubber composition is notparticularly limited, and can appropriately be chosen depending upon theshape and size of rubber vulcanizate. A vulcanizable rubber compositioncan be filled in a mold and heated whereby molding and crosslinking areconcurrently carried out, or an uncrosslinked rubber composition can bepreviously molded and the molded product can be heated to be therebycrosslinked. The crosslinking Is carried out preferably at a temperaturein the range of 120° C. to 200° C., more preferably 140° C. to 180° C.The crosslinking time is usually in the range of about 1 to 120 minutes.

[0105] The invention will now be specifically described by the followingworking examples. Parts and % in the following production examples,examples and comparative examples are by weight unless otherwisespecified.

[0106] The properties of a raw material ingredient for rubber, a rubbercomposition and a rubber vulcanizate were determined as follows.

(1) Coagulability of Latex

[0107] A latex was diluted with water to adjust the solid concentrationto 10%. A coagulating vessel was charged with 300 parts of an aqueoussolution containing 0.06 part of calcium chloride and 0.006 part of apolymeric flocculant (cationic condensate type resin “HISET CA™”available from Dai-ichi Kogyo Seiyaku Co. Ltd.), and then, 100 parts ofthe diluted latex was gradually added, while being stirred for mixing,in a conventional manner whereby the latex was coagulated. Thetemperature of a coagulation bath within the coagulation vessel wascontrolled to a temperature in the range of 55° C. to 65° C. The pHvalue of the coagulation bath was adjusted to a value in the range of 2to 3 by adding an aqueous 5% sulfuric acid solution. The state ofcoagulation was observed and expressed by the following five ratingindexes. The larger the index, the more excellent the coagulability.

[0108] 1: Even if the amounts of calcium chloride and polymericflocculent are increased to 1.5 times of the above-recited amounts atthe coagulation step, crumbs have a large sizes and are intenseopaque-white.

[0109] 2: Crumbs have a large size and are opaque-white to a smallextent.

[0110] 3: Crumbs have an acceptable size and are opaque-white to aslight extent. The opaque-whiteness gradually disappears.

[0111] 4: Even if the amounts of calcium chloride and polymericflocculant are decreased to 0.7 times of the above-recited amounts atthe coagulation step, crumbs have an acceptable size and areopaque-white only to a negligible extent.

[0112] 5: Even if calcium chloride is not added at the coagulation step,crumbs have an acceptable size and are opaque-white only to negligibleextent.

(2) Stickiness of Crumbs

[0113] A latex was coagulated under the same conditions as mentionedabove for evaluation of coagulability of latex. The state of coagulationand stickiness was observed and expressed by the following four ratingindexes. The larger the index, the more satisfactory the stickiness ofcrumbs.

[0114] 1: Crumbs are sticky and deposited onto rotational parts of astirrer and the inner wall of a coagulation vessel, and enlarged crumbsaccumulate on the bottom of the vessel.

[0115] 2: Crumbs are sticky and deposited onto rotational parts of astirrer and the inner wall of a coagulation vessel, and a large numberof somewhat enlarged crumbs are observed.

[0116] 3: A precipitate of somewhat enlarged crumbs is observed, butdeposition of crumbs onto rotational parts of a stirrer and the innerwall of a coagulation vessel was not observed.

[0117] 4: Crumbs have an acceptable size and deposition thereof ontorotational parts of a stirrer and the inner wall of a coagulation vesselwas not observed in the least.

(3) Particle Diameter of Rubber Gel Particles

[0118] A latex was diluted with water to adjust the solid concentrationto about 0.01%. The diluted latex was dropped onto a mesh forobservation by a transmission electron microscope. The dropped latex wasdyed and fixed with osmium tetroxide vapor, and then water wasevaporated therefrom to prepare a sample for observation. The sample wasobserved by a transmission electron microscope of 20,000× to 50,000×magnification. Particle diameter (unit: nm) was observed on 100particles, and a weight average particle diameter was calculated.

(4) Amount of Styrene Units

[0119] Amount of styrene units bound in a copolymer was measuredaccording to JIS-K 6383. It is noted, however, that, when a copolymercomprises divinylbenzene units in addition to styrene units, the sum ofstyrene units and divinylbenzene units is unavoidably determined.

(5) Toluene Swelling Index

[0120] The swelling index of a rubber gel as measured in toluene wasdetermined as follows. 250 g of a sample rubber gel was placed in 25 mlof toluene and the mixture was shaken for 24 hours whereby the rubbergel was swollen. The swollen rubber gel was subjected to centrifugingunder a centrifugal force of 430,000 m/sec² by a centrifugal separator.The as-centrifuged rubber gel was weighed in a wet state (weight “a”),and then dried at 70° C. to a constant weight. The dried gel was weighed(weight “b”). The toluene swelling index Is expressed by the ratio (a/b)of weight “a” of wet rubber gel to weight “b” of dried rubber gel.

(6) Mooney Viscosity:

[0121] The Mooney viscosity (ML₁₊₄, 100° C.) of a raw material rubberwas measured according to JIS-K 6300.

(7) Mechanical Properties of Rubber Vulcanizate

[0122] Tensile strength and elongation of a rubber vulcanizate weremeasured according to JIS-K 6301.

(8) Abrasion Resistance Index

[0123] The abrasion resistance was measured by a Pico abrasion machineaccording to JIS-K 6264, and expressed in terms of an index as theabrasion resistance obtained in Comparative Example 1 being 100. Thelarger the abrasion resistance index, the more satisfactory the abrasionresistance.

(9) Low Heat Build Up

[0124] tanδ value was measured at a temperature of 60° C. underconditions of a twist of 0.5% and a frequency of 20 Hz by “RDA-II™”available from Rheometric Scientific Inc. The smaller the tanδ (60° C.)value, the more satisfactory the low heat-build up. The tanδ (60° C.)value is expressed in terms of an index as the tanδ (60° C.) valueobtained in Comparative Example 1 being 100. The larger the index oftanδ (60° C.) value, the more satisfactory the low heat-build up.

EXAMPLE 1 Production of Conjugated Diene Rubber Gel I

[0125] A pressure reaction vessel was charged with 200 parts of water,4.5 parts of a mixture of disproportionated potassium rosinate andsodium fatty acid as an emulsifier, 0.1 part of potassium chloride, and,a monomer mixture and a chain transfer agent (t-dodecyl mercaptan),shown in Table 1. The temperature of the content was controlled to 12°C. while being stirred, and 0.1 part of cumene hydroperoxide, 0.2 partof sodium formaldehyde sulfoxylate and 0.01 part of ferric sulfate, as aradical polymerization initiator, were added to the content to initiatepolymerization.

[0126] The polymerization reaction was continued at 12° C. until theconversion reached 70%, and then 0.1 part of diethylhydroxylamine wasadded to terminate the polymerization reaction. A part of thethus-obtained latex was taken and subjected to gas chromatography. Theamounts of unreacted monomers were determined from previously preparedcalibration curves. The amount of the monomer units constituting acopolymer was determined as a difference between the amount of chargedmonomer and the amount of unreacted monomer, The results are shown inTable 1.

[0127] Then the latex was heated and subjected to steam distillation atabout 70° C. under a reduced pressure to recover unreacted monomers. To100 parts of the thus-produced copolymer, 2 parts of2,6-di-t-butyl-4-methylphenol as an antioxidant was added. A part of thethus-obtained latex was taken to determine its weight average particlediameter. The results are shown in Table 1.

[0128] Then the latex was placed in a sodium chloride/sulfuric acidsolution whereby the latex was coagulated. The thus-formed crumbs weretaken, thoroughly washed with water, and then dried at 50° C. under areduced pressure to give a conjugated diene rubber gel I. The amount ofstyrene units and toluene swelling index of conjugated diene rubber gelI are shown in Table 1.

EXAMPLES 2-6 Production of Conjugated Diene Rubber Gels II to VI

[0129] Conjugated diene rubber gels II to M were produced by the sameprocedures as those described in Example 1 except that monomer mixtureshaving a composition shown in Table 1 and a chain transfer agent shownin Table 1 were used. The properties of the rubber gels are shown inTable 1.

EXAMPLE 7 Production of Conjugated Diene Rubber Gel VII

[0130] Conjugated diene rubber gel VII was produced by the sameprocedures as those described in Example 1 except that a monomer mixturehaving a composition shown in Table 1 and a chain transfer agent shownin Table 1 were used, 0.2 part of potassium sulfate was used as theradical polymerization initiator, and the polymerization was terminatedat a conversion of 92% with all other conditions remaining the same. Theproperties of rubber gel VII are shown in Table 1.

[0131] Conjugated diene rubber gels I to VII contained only a negligibleamount of rubber ingredient soluble in toluene.

COMPARATIVE PRODUCTION EXAMPLE 1 Production of Conjugated Diene Rubber I

[0132] Conjugated diene rubber gel I was produced by the same proceduresas those described in Example 1 except that a monomer mixture having acomposition shown in Table 1 was used. The amount of styrene units andtoluene swelling index of conjugated diene rubber I are shown inTable 1. Conjugated diene rubber I was substantially free from gel, andhence, did not exhibit any significant measured value for tolueneswelling index. TABLE 1 Pd. Examples Ex. 1 2 3 4 5 6 7 1 ConjugatedDiene Rubber Gel I II III IV V VI VII — Conjugated Diene Rubber — — — —— — — I Monomer Mixture (parts) Butadiene 99.7 92.5 82 77 70 82 84.6 70Styrene — 7.2 17.7 22.7 29.7 16 15 30 Divinylbenzene 0.3 0.3 0.3 0.3 0.32 0.4 — Chain Transfer Agent t-Dodecyl mercaptan 0.3 0.3 0.3 0.3 0.3 0.30.3 0.3 Polymerization Temperature (° C.) 12 12 12 12 12 12 50 12Polymerization Conversion at 70 70 70 70 70 70 92 70 Termination ofPolymerization (%) Composition of Copolymer (%) Butadiene Units 99.7 9588 84 76 88 88 76 Styrene Units — 4.8 11.8 15.8 23.8 10.4 11.7 24Divinylbenzene Units 0.2 0.2 0.2 0.2 0.2 1.6 0.3 — Weight AverageParticle Diameter (nm) 90 88 87 84 88 89 85 — Styrene Units (%) 0.2 5 1216 24 12 12 24 Toluene Swelling Index 23 25 22 20 23 4 25 — MooneyViscosity — — — — — — — 46

[0133] As shown in Examples 1 to 7 in Table 1, by the process of thepresent invention for producing a conjugated diene rubber gel, aconjugated diene rubber gel having a desired polymer composition and adesired toluene swelling index can easily be produced with enhancedproductivity. In contrast, if a conjugated diene rubber gel having adesired toluene swelling index is produced from a conjugated dienerubber latex having no gel structure shown in Comparative ProductionExample 1, it is indispensable, after the removal of unreacted monomersfrom a latex as obtained by termination of polymerization, to add aperoxide to the latex and heat-treat the peroxide-added latex.

EXAMPLES 8-11 AND COMPARATIVE EXAMPLES 1-4 Production of RubberVulcanizates and Evaluation Thereof

[0134] 100 parts of a rubber composition comprising the rubberingredients shown in Table 1 was mixed with 40 parts of carbon black(“Seast SO™” available from Tokai Carbon K.K.), 3 parts of zinc oxide, 2parts of stearic acid and 2 parts ofN-(1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine as an antioxidant, andthe mixture was kneaded at 120° C. for 6 minutes by a Banbury mixer.Then the kneaded mixture was kneaded together with 1.1 parts of sulfurand 0.9 part of N-t-butyl-2-benzothiazyl sulfenamide as a crosslinkingaccelerator at 50° C. by an open roll to give a rubber composition. Therubber composition was press-cured at 160° C. for 12 minutes to give arubber vulcanizate. The properties of the rubber vulcanizate wereevaluated, and the results are shown in Table 2.

[0135] As seen from Table 2, a rubber vulcanizate made from a conjugateddiene rubber gel containing a minor amount of styrene units has poorabrasion resistance (Comparative Example 2). A rubber vulcanizate madefrom a conjugated diene rubber gel containing a large amount of styreneunits is not satisfactory in heat-build up (Comparative Example 3). Arubber vulcanizate made from a conjugated diene rubber gel having asmall toluene swelling index has a drastically reduced elongation and apoor abrasion resistance (Comparative Example 4). TABLE 2 ExamplesComparative Examples 8 9 10 11 1 2 3 4 Composition of Rubber Ingredients(parts) Natural Rubber 70 70 70 70 70 70 70 70 Conjugated Diene RubberGel I — — — — — 30 — — Conjugated Diene Rubber Gel II 30 — — — — — — —Conjugated Diene Rubber Gel III — 30 — — — — — — Conjugated Diene RubberGel IV — — 30 — — — — — Conjugated Diene Rubber Gel V — — — — — — 30 —Conjugated Diene Rubber Gel VI — — — — — — — 30 Conjugated Diene RubberGel VII — — — 30 — — — — Conjugated Diane Rubber I — — — — 30 — — —Physical Properties of Rubber Vulcanizate Tensile Strength (MPa) 23 2727 24 23 23 26 22 Elongation (%) 440 500 490 510 450 420 420 320Abrasion Resistance Index 104 132 127 119 100 95 102 96 tanδ (60° C.)Index 112 110 107 104 100 115 98 113

[0136] In contrast to the rubber vulcanizates of the comparativeexamples, rubber vulcanizates obtained in Examples 8-11 of the presentinvention exhibit high abrasion resistance and low heat-build up withoutdeterioration of mechanical properties. As seen from the comparison ofExample 9 with Example 11, the rubber vulcanizate made of a conjugateddiene rubber gel as obtained by polymerizing at a lower temperature andterminating the polymerization at a conversion of 70% (Example 9) issuperior to the rubber vulcanizate obtained in Example 11.

EXAMPLE 12 Production of Conjugated Diene Copolymer Rubber I

[0137] A pressure reaction vessel was charged with 200 parts of water,4.5 parts of a mixture of disproportionated potassium rosinate andsodium fatty acid as an emulsifier, 0.1 part of potassium chloride, and,a monomer mixture and a chain transfer agent (t-dodecyl mercaptan),shown in Table 3. The temperature of the content was controlled to 10°C. while being stirred, and 0.1 part of cumene hydroperoxide, 0.2 partof sodium formaldehyde sulfoxylate and 0.01 part of ferric sulfate, as aradical polymerization initiator, were added to the content to initiatepolymerization.

[0138] The polymerization reaction was continued at 10° C. until theconversion reached 70%, and then 0.1 part of diethylhydroxylamine wasadded to terminate the polymerization reaction. Then the thus-obtainedlatex was heated and subjected to steam distillation at about 70° C.under a reduced pressure to recover unreacted monomers. To 100 parts ofthe thus-produced copolymer, 2 parts of 2,6-di-t-butyl-4-methylphenol asan antioxidant was added.

[0139] Then water was added to the latex to adjust its solidconcentration to 10%. Coagulability of the latex and stickiness ofcrumbs were evaluated by the above-mentioned methods. The crumbsobtained were thoroughly washed with warm water at 60° C. and thendrained. The drained crumbs were dried at 80° C. by a warm air dryer togive conjugated diene copolymer rubber I. The amount of styrene units inthe copolymer rubber I and the Mooney viscosity thereof are shown inTable 3.

EXAMPLES 13-15 Production of Conjugated Diene Copolymer Rubbers II to IV

[0140] Conjugated diene copolymer rubbers II to IV were produced by thesame procedures as those described in Example 1 except that monomermixtures having a composition shown in Table 3 and a chain transferagent shown in Table 3 were used with all other conditions remaining thesame. The coagulability, stickiness of crumbs, amount of styrene unitsand Mooney viscosity of the copolymer rubbers are shown in Table 3.

COMPARATIVE EXAMPLES 5-7 Production of Conjugated Diene CopolymerRubbers V to VII

[0141] Conjugated diene copolymer rubbers V, VI and VII were produced bythe same procedures as those described in Example 1 except that monomermixtures having a composition shown in Table 3 and a chain transferagent shown in Table 3 were used with all other conditions remaining thesame. The coagulability, stickiness of crumbs, amount of styrene unitsand Mooney viscosity of the copolymer rubbers are shown in Table 3.

REFERENCE EXAMPLES 1 and 2 Production of Conjugated Diene CopolymerRubbers VIII and IX

[0142] Conjugated diene copolymer rubbers VIII and IX were produced bythe same procedures as those described in Example 1 except that monomermixtures having a composition shown in Table 3 and a chain transferagent shown in Table 3 were used with all other conditions remaining thesame. The coagulability, stickiness of crumbs, amount of styrene unitsand Mooney viscosity of the copolymer rubbers are shown in Table 3.

[0143] The conjugated diene copolymer rubbers I to IX had a weightaverage particle diameter in the range of 80 to 100 nm. TABLE 3 ExamplesComp. Ex. Ref. Ex. 12 13 14 15 5 6 7 1 2 Conjugated Diene I II III IV VVI VII VIII IX Rubber Gel Monomer Mixture (parts) Butadiene 45 45 45 6245 45 62 70 70 Styrene 54.7 54.9 53 37.7 55 55 38 29.7 30 Divinylbenzene0.3 0.1 2 0.3 — — — 0.3 — Chain Transfer Agent t-Dodecyl 0.3 0.5 0.3 0.30.3 0.06 0.3 0.3 0.3 mercaptan Coagulability 5 4 5 4 1 1 3 4 3 (Index)Adherence of 4 3 4 4 1 2 2 4 4 Crumbs (Index) Styrene Units (%) 45 45 4535 45 45 35 24 24 Mooney Viscosity 105 163 53 122 46 120 44 82 41

[0144] As seen from Comparative Examples 5 and 6, when copolymer rubberscomprise a large amount (45%) of styrene units. the coagulability andstickiness of crumbs were poor, and thus, the production of thecopolymer rubbers is difficult. When a copolymer rubber comprisesstyrene units of 35%, the coagulability and stickiness of crumbs werepoor (Comparative Example 7). When a copolymer rubber comprises 24% ofstyrene units, the coagulability and stickiness of crumbs are good(Reference Example 1). But, when a copolymer rubber produced without useof a crosslinking monomer comprises 24% of styrene units, thecoagulability and stickiness of crumbs are somewhat similarly good(Reference Example 2). Therefore, the results obtained in ReferenceExample 1 are deemed not to be conspicuous.

[0145] In contrast, by the second production process of the presentinvention, butadiene-styrene copolymer rubbers comprising a large amountof styrene units and exhibiting good coagulability and satisfactorilyreduced stickiness of crumbs can be obtained as seen from Examples 12 to15.

Evaluation of Drying Characteristics of Crumb

[0146] Drying characteristics of crumbs were evaluated by the followingtesting method.

[0147] After draining, 100 g of a wet coagulated crumb of a conjugateddiene copolymer rubber is taken in a cage with a mesh, and the cage isplaced in a warm air dryer where the crumb is dried at 80° C. for 3hours. The moisture content in the dried copolymer rubber is determinedas weight reduction in weight % as measured when the dried copolymerrubber is subjected to vacuum drying at 125° C. for 30 minutes.

[0148] Copolymer rubber I obtained in Example 1 exhibited a moisturecontent of 0.2% by weight. In contrast, copolymer rubber V obtained inComparative Example 1 exhibited a moisture content of 2.2% by weight andoccurrence of wet spots was observed These test results show that theconjugated diene copolymer rubber produced by the second productionprocess of the present invention has good drying characteristics.

Industrial Applicability

[0149] The novel conjugated diene copolymer gel of the present inventiongives a rubber composition A rubber vulcanizate made from a rubbercomposition comprising the conjugated diene copolymer gel, and a rubbervulcanizate made from a rubber composition comprising the conjugateddiene copolymer gel and a rubber capable of being crosslinked withsulfur, can be widely used as a component of, for example, tires, cablecoverings, hoses, transmission belts, conveyor belts, roll covers, shoesoles, sealing rings and vibration rubber insulators.

[0150] The rubber vulcanizate made from a rubber composition comprisingthe conjugated diene copolymer gel, and the rubber vulcanizate made froma rubber composition comprising the conjugated diene copolymer gel and arubber capable of being crosslinked with sulfur, exhibit enhancedabrasion resistance and reduced heat-build up without deterioration ofmechanical properties. Therefore, these rubber vulcanizates are suitablefor tires, especially as components of sidewall, bead and under-tread.

[0151] According to the first production process of the presentinvention wherein a conjugated diene rubber gel is produced by anemulsion copolymerization without use of or using a minor amount of anaromatic vinyl monomer, a conjugated diene rubber gel exhibiting theabove-mentioned properties and having a toluene swelling index notlarger than 70 can easily be obtained with high productivity.

[0152] According to the second production process of the presentinvention wherein a conjugated diene rubber is produced by an emulsioncopolymerization using a relatively large amount of an aromatic vinylmonomer, a conjugated diene-aromatic vinyl copolymer rubbercharacterized in that a crumb exhibits desirably reduced stickiness anda latex exhibits good coagulability can be obtained.

1. A process for producing a conjugated diene rubber gel having aswelling index of not larger than 70 as measured in toluene,characterized in that a monomer mixture comprising 50% to 99.9% byweight of a conjugated diene monomer, 0% to 30% by weight of an aromaticvinyl monomer, 0% to 20% by weight of other ethylenically unsaturatedmonomer and 0.1% to 20% by weight of a crosslinking monomer iscopolymerized by an emulsion polymerization procedure.
 2. The processfor producing a conjugated diene rubber gel according to claim 1,wherein the emulsion-copolymerization reaction is terminated at aconversion in the range of 50% to 90%.
 3. The process for producing aconjugated diene rubber gel according to claim 1, wherein theemulsion-copolymerization reaction is carried out at a temperature inthe range of 3° C. to 30° C. and terminated at a conversion in the rangeof 60% to 85%.
 4. A process for producing a conjugated diene-aromaticvinyl copolymer rubber, characterized in that a momomer mixturecomprising 15% to 69.8% by weight of a conjugated diene monomer, 30.1%to 65% by weight of an aromatic vinyl monomer, 0% to 20% by weight ofother ethylenically unsaturated monomer and 0.1% to 20% by weight of acrosslinking monomer is copolymerized by an emulsion polymerizationprocedure.
 5. The process for producing a conjugated diene-aromaticvinyl copolymer rubber according to claim 4, wherein theemulsion-copolymerization reaction is carried out at a temperature inthe range of −5° C. to 80° C. and terminated at a conversion in therange of 50% to 90%.